The radio frequency (RF) spectrum is a limited commodity. Only a small portion of the spectrum can be assigned to each communications industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum. Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. The CDMA wireless communications system spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal. A signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz.
All of the wireless access terminals, including both mobile stations and fixed terminals, that communicate in a CDMA system transmit on the same frequency. Therefore, in order for the base station to identify the wireless access terminals, each wireless access terminal is assigned a unique pseudo-random (PN) long spreading code that identifies that particular wireless access terminal to the wireless network. Typically, each long code is generated using the electronic serial number (ESN) of each mobile station or fixed terminal. The ESN for each wireless access terminal is unique to that wireless access terminal.
In some CDMA wireless networks, during the transmission of user data from a wireless access terminal to a base station (i.e., reverse channel traffic), the user data are grouped into 20 millisecond (msec.) frames. All user data transmitted on the reverse channel are convolutionally encoded and block interleaved to form a baseband signal. In a preferred embodiment, the baseband signal is then modulated by an M-ary orthogonal modulation in which each N-bit data sequence or symbol is replaced by an orthogonal modulation code sequence of length M=2N. The M-ary modulated signal is then spread using a long code based on the ESN data and then separated into an in-phase (I) component and a quadrature (Q) component prior to quadrature modulation of an RF carrier and transmission.
Next, the I-component is modulated by a zero-offset short pseudo-random noise (I-PN) binary code sequence. The Q-component is modulated by a zero-offset short pseudo-random noise (Q-PN) binary code sequence. In an alternate embodiment, the quadrature binary sequence may be offset by one-half of a binary chip time. Those skilled in the art will recognize that the in-phase component and the quadrature component are used for quadrature phase shift keying (QPSK) modulation of an RF carrier prior to transmission. Those skilled in the art will also recognize that the access terminal may use binary phase shift keying (BPSK) modulation, quadrature amplitude modulation (QAM) or other digital modulation format for modulation of an RF carrier for transmission of the data signals prior to transmission.
For IS-95 and IS-2000 based systems, the M-ary modulation uses M=26 orthogonal binary sequences for 6-bit encoding. In other words, six (N=6) bit blocks (or symbols) of the encoded and interleaved baseband signal are represented by one of 26 (i.e., 64) unique codes. In 64-ary modulation used in current CDMA systems, one of 64 possible Walsh codes is transmitted for each group of six (6) coded bits of the baseband signal. Within a Walsh function, sixty-four (64) Walsh chips are transmitted. The particular Walsh function is selected according to the relation:Walsh Function=c0+2c1+4c2+8c3+16c4+32c5  (1)where c5 represents the last coded bit and co represents the first coded bit in the six-bit group of baseband data. Upon receipt of the transmitted signal from the access terminal, the base station performs the inverse of this sequence to detect the transmitted user baseband data bits.
Those skilled in the art will recognize that instead of M-ary modulation described previously, the baseband signal may be spread with an M-bit Walsh code, a quasi-orthogonal function or a turbo code prior to up-conversion and modulation of an RF carrier for transmission.
For multipath propagation, the base station may employ spatial diversity reception with two independent receive paths to receive a fading signal from the kth access terminal. In a preferred embodiment, diversity reception comprises two or more antennas separated by a distance equal to ten (10) or more wavelengths of the received RF signal. Those skilled in the art will recognize that signals arriving at the two or more antennas from the same source are un-correlated with antennas separations of ten (10) or more wavelengths. That is, if the signal received by one antenna is faded, the signal received by another antenna is not faded. Each antenna is connected to receive circuitry that performs separate despreading, M-ary demodulation, de-interleaving and convolutional decoding functional blocks for processing each multipath signal received by the base station. If the signal from the access terminal to one of the antennas undergoes a fade, a signal on the radio path from the access terminal to the second antenna may not have be in a fade condition. A selector circuit selects the best signal from the multiple diversity receive circuits to mitigate the affects of fading.
In conventional CDMA systems, the M-ary demodulator for demodulation of the signal from the kth access terminal consists of a bank of matched filters needed to detect one out of the M possible N-bit data symbols. A separate bank of matched filters is required for processing the signal received on each path. Each matched filter consists of M stages for processing the M modulation symbol bits to detect one out of the M possible N-bit data symbols. This greatly increases the number of ASIC gates or DSP processing (instructions per second) to detect the one of M=2N possible N-bit data symbols (patterns) in demodulating the M-ary modulated signal.
Therefore, there is a need for an M-ary demodulator that reduces the signal processing complexity required to perform M-ary demodulation of M-ary modulated data symbols. In particular, there is a need for an M-ary demodulator that does not require a separate bank of matched filters on each received signal path, wherein each matched filter consists of M stages for processing the M modulation symbol bits.